System for application of powder coatings to electrically non-conductive elements

ABSTRACT

A method and system for powder coating non electrically conductive elements, preferably brake pads. A pre-treatment station is upstream of an electrostatic powder coating deposition station and a baking station for melting and polymerizing the powder coating in order to form a coating layer on a surface to be coated. The pre-treatment station causes the elements to be coated to conduct electrically by uniformly wetting said elements by means of creating poorly mineralized water covalent bonds on at least one surface to be coated, in an amount aimed at producing a measurable weight increase in the non electrically conductive elements, which then causes them to conduct electrically. The water adsorbed and/or deposited is subsequently eliminated within the baking station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/442,430, filed May 13, 2015, which is a national stageapplication under 35 U.S.C. § 371 of International Application No.PCT/IB2013/060115, filed Nov. 13, 2013, which claims priority of ItalianPatent Application No. TO2012A000981, filed Nov. 13, 2012. The entirecontents of each noted application are herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a method and process for powder coatingnon electrically conductive elements, and in particular brake pads madewith NAO (“Non Asbestos Organic” i.e. asbestos-free organic friction)friction materials.

TECHNICAL BACKGROUND

Friction materials utilized as the lining in drum brake shoes of drumbrakes and in the brake pads in automobile disk brakes and other devices(for example in clutch plates) are manufactured with a compound(mixture) comprising a fibrous or fiber material, an organic binder(usually a phenolic synthetic resin) and a bulk or “filler”. Instead ofasbestos as the fiber material, which has been legally banned as asubstance dangerous to the environment, mixtures of other organic andinorganic materials are utilized, such as rock wool, aramid and carbonfibers, metal fibers or powder such as copper, tin, iron, aluminum, andother metals or metal alloys such as bronze or brass. EP1227262 forexample, indicates the use of a friction material of the above describedtype containing approximately 10% copper fibers by volume, between 0.1and 15% tin and/or tin sulfide by volume, and between 4 and 9% bronzefibers by volume.

NAO compounds, due to their composition, have an electric conductivitynot sufficient to ensure a good painting thereof; things are even worseif the copper is eliminated (due to environment care compounds withoutcopper are more and more requested).

Even though in brake pads the compound is manufactured as a plaque orpad assembled on a metal plate in order to form the actual brake pad,painting brake pads made with non electrically conductive materialspresents numerous problems. In particular, it is presently impossible toutilize currently used powder coating plants built to coat/paint brakepads that are electrically conductive.

More generally it is also known that in order to powder coating (apainting technology requiring use of electrostatic charges) nonelectrically conductive elements such as for example mechanicalcomponents manufactured with plastic polymers, a conductive primer isapplied to the surface of the non conductive element to be painted.However, conductive primers are based on organic solvents which arenoxious and dangerous to the environment.

Attempts to powder coat brake pads made with non conductive compoundshave not been successful up to now; this is because, when at allpossible, (thanks to the presence of the metal base support for theplaque in a non conductive compound), the preparation, finish, andthickness are not satisfactory: brake pads manufactured using thismethod are therefore unable to pass standard corrosion tests.

The only alternative available therefore is the use of other coatingtechnologies, which are however more expensive and require largespecially devised plants that call for an unacceptable investmentconsidering present manufacturing volumes.

BRIEF DESCRIPTION

The object of the present invention is to provide a powder coatingmethod and process for non electrically conductive elements, and inparticular NAO brake pads, which will lead to optimal application,coverage and surface finishing, comparable to that which can be achievedwith plants that are currently in use and that have been designed fortreating electrically conductive brake pads manufactured using metalcontaining compounds; all of this while utilizing presently existingplants with minimal modifications and without the use of conductiveprimers.

The invention therefore relates to a powder coating method for nonelectrically conductive elements, in particular brake pads. Theinvention further relates to a powder coating plant for treating nonelectrically conductive elements, in particular brake pads.

According to a main aspect of the invention, upstream of anelectrostatically charged painting powder coating/deposition station andof a baking station used to melt and polymerize the coating powder inorder to form a coating layer on the surface to be coated of the nonconductive elements, a pre-treatment station is present in order to makethe elements to be coated temporarily electrically conductive byuniformly wetting the same by adsorption and/or deposition of water,preferably poorly mineralized water, on at least the aforementionedsurface to be coated with an amount that is sufficient to produce ameasurable weight increase in the non conductive elements, which willthen cause said elements to become electrically conductive. The adsorbedand/or deposited water is subsequently eliminated within the bakingstation.

Within the scope of the present invention, “poorly mineralized water” or“low mineral content water” shall be understood to be water which isdevoid of ions which would be potentially chemically reactive with ironcompounds and that has a dry residue and an electric conductivity whichare quantitatively comparable to those of bottled water.

In particular, according to an aspect of the invention, the water to beutilized according to the method of the invention and the plant toimplement thereof must possess a chemical composition devoid of ionswhich would be potentially chemically reactive with iron compounds andsuch that the water possesses a value of the specific electricalconductivity measured at 20° C. comprised between 1 and 5,000 μS/cm andpreferably comprised between 10 and 700 μS/cm [in the Metric System “S”is the symbol for Siemens].

Furthermore, according to the applicant's technician's findings thewater must be adsorbed and/or deposited on each non electricallyconductive element to be made conductive in an amount that will cause aweight increase in each electrically non conductive element of between0.15% and 0.30%.

In this manner the electric resistance of brake pads manufactured, withasbestos-free organic friction compounds changes from an order ofmagnitude of 1,000,000 MSΩ (Mega Ohm) before pre-treatment, to valuesaround 0.011 MΩ after pre-treatment, and then back to pre-treatmentvalues at the end of the baking phase, which is commonly carried out ina tunnel oven. Hence pre-treated brake pads can be coated by means oftraditional powder coating methods by depositing the coating powderelectrostatically by means of the same steps already in use for brakepads manufactured with electrically conductive compounds.

According to additional aspects of the invention, the plant forimplementing the method of the invention may be realized according totwo separate embodiments.

In a first embodiment the pre-treatment station comprises a superheatedsteam generator operating with a low mineral content water, for instancewell water; a plurality of delivery nozzles for a superheated steamtowards the non electrically conductive elements; an air/steam specificvolume of between 15 and 30 m³/kg aimed at the non electricallyconductive elements, when the elements are heated on a transportmechanism upstream of the electrostatic powder coating station; andcooling means where the non electrically conductive elements are drivenby the transport mechanism downstream of the air/steam mixing means andimmediately upstream of the electrostatic powder coating station.

In a second preferred embodiment, the pre-treatment station comprises amotorized roller rack for tidily conveying the non electricallyconductive elements; a plurality of nozzles for delivery high pressurejets of low mineral content water towards the non electricallyconductive elements, said nozzles being suitable for creating a mist allaround the non electrically conductive elements; at least one supplypump for the nozzles; and an extraction hood facing the nozzles.

According to this embodiment, the entire pre-treatment station laid outalong its longitudinal axis, that is to say in the direction ofadvancement of the brake pads to be coated, occupies overall a sectionof only 45 mm (equal to the length of the motorized roller rack),implements the pre-treatment stage in only 10 seconds, and withnegligible power consumption (about 1 KW).

In particular these results are obtained by means of two constructivefeatures: on one hand the nozzles are aligned in line to one anotherunderneath the motorized roller rack, so that their jets are aimed frombelow upwards at a predetermined vertical distance from the motorizedroller rack, leaving an empty space, possibly adjustable, between therack and the nozzles; and on the other hand the extraction hood isdefined by a tubular element provided with a straight collection slitfacing the nozzles in the same direction of alignment as the nozzlesthemselves. As a matter of fact, it has been found that when these twoconstructive features are present at the same time, they have asynergistic effect that permits regulation of the amount of poorlymineralized water deposited on the brake pad and that wets the compoundto be treated with an extremely high degree of precision. This is of theessence in order to achieve the desired electrical conductivity withoutsoaking too much the brake pads.

An additional advantage of the system according to the invention is thatit is capable of coating both, NAO asbestos-free organic brake pads andlow steel metallic brake pads by simply turning off the pre-treatmentstation. It is therefore possible to alternate from one type of compoundto the other on the same production line without performing anymodifications.

Finally, the plant and method of the invention are completelyenvironmentally friendly.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages of the present invention will beapparent from the following description of the two non limitingembodiments thereof given exclusively for exemplification purposes withrespect to the enclosed drawings, where:

FIG. 1 depicts a schematic lateral elevation view, partially in alongitudinal cross-section, of a powder coating plant implementedaccording to the invention;

FIG. 2 depicts an enlarged scale schematic view in cross-section of acomponent in the plant in FIG. 1;

FIG. 3 depicts an enlarged scale perspective view of a second componentof the plant in FIG. 1;

FIG. 4 depicts a schematic lateral elevation view, partially in alongitudinal cross-section, of a second embodiment of the powder coatingplant implemented according to the invention; and

FIGS. 5 and 6 depict two charts displaying the weight and electricalconductivity variation of asbestos-free organic friction compound brakepads treated according to the method of the invention.

DETAILED DESCRIPTION

With reference to FIG. 1, is depicted a powder coating plant, overallindicated with reference number 1, for painting non electricallyconductive elements 2, in particular brake pads manufactured withasbestos-free organic friction compounds, known in the art and thereforenot shown in detail but only illustrated as blocks.

Said type of brake pads are initially manufactured by molding thecompound at a temperature of between 130 and 200° C. in order to createa plaque or pad, which are subsequently cured by thermal treatment,assembled onto an iron metal support, and finally painted together withthe support.

NAO brake pad compounds obtained in this manner however, usingpreviously existing technologies, cannot be powder coated but only spraypainted, with all the associated issues, including protection of theenvironment, that this implies.

According to the invention, on the contrary, the powder coating plant 1is utilized, which in general comprises a pre-treatment station 3, wherenon electrically conductive elements 2 are made to be temporarilyelectrically conductive in the manner that will be described hereinafterand are therefore “transformed” into electrically conductive elements 2b, a station 4, of a type known in the art and therefore illustratedonly schematically, where coating powders 5 are appliedelectrostatically onto elements 2 b, a melting and polymerizationstation 6 for the coating powders 5, preferably defined by a baking ovenof the tunnel type known in the art and therefore illustrated onlyschematically for simplicity, and at least one transport mechanism 7suitable for tidily transporting non electrically conductive elements 2to be coated in series along stations 3, 4, and 6, in order to traversethe same and finally produce non electrically conductive elements 2provided with at least one surface 8 to be coated (which may extend tothe entire external surface of elements 2 or to part only thereof as thecase may be), a coating layer 9 made of the coating powders 5 melted andpolymerized in oven 6 at the temperature common for powder coatingprocesses (in general around 200-220° C.).

In the depicted non limiting example, plant 1 comprises a transportdevice 7 traversing stations 4 and 6, and which places the coatedelements 2 onto a table 10, and a second device 7 supplying elements 2to station 3, located upstream of stations 4 and 6.

Hereinafter the terms “upstream” and “downstream” are understood asreferring to the direction of advancement D of non electricallyconductive elements 2 along plant 1, and more specifically stations 3,4, and 6, indicated by the arrows in FIG. 1.

According to the invention, pre-treatment station 3 comprises a means 11for depositing and/or adsorbing poorly mineralized water onto at leastthe surface 8 to be coated of the non electrically conductive elements2, and preferably on each entire non electrically conductive element 2,in order to cause a measurable weight increase in the non electricallyconductive elements 2, corresponding to an amount of low mineral contentwater retained on the same (at least in correspondence with the surface8) thus causing said elements 2 to temporarily electrically conduct,“transforming them” into elements 2 b.

As previously stated, “poorly mineralized water” is to be understood aswater having a composition devoid of ions that would be potentiallychemically reactive with iron compounds (which constitute the supportfor the asbestos-free organic friction compound of brake pads 2), andhence devoid of ions such as Cl⁻, and having an amount of dry residueand an electric conductivity quantitatively comparable to those of thebottled water. The poorly mineralized water that may be used in plant 1according to the invention must nonetheless possess a chemicalcomposition such that it has a specific conductivity measured at 20° C.comprised between 1 and 5,000 μS/cm and preferably between 10 and 700μS/cm.

Furthermore, still according to the invention, melting andpolymerization station 6 for coating powders 5 must be suitable not onlyfor melting and polymerizing powders 5, but also for eliminating atleast part (in fact substantially all) of the water previously adsorbedand or deposited onto non electrically conductive elements 2. Takinginto account that station 6 comprises a tunnel oven known within the artwhere temperatures in the range of 200° C. are reached, this lastcharacteristic is built-in. Nonetheless, said characteristic isnecessary within the scope of the invention, and hence precludes the useof melting and polymerization stations where there is no certainty ofeliminating the water retained upon/within the elements 2.

According to the embodiment illustrated in FIG. 1, pre-treatment station3 of plant 1 comprises in combination: a rack 12 comprising a pluralityof, for example four, motorized rollers 13 displaced each from the otherin direction D along which the non electrically conductive elements 2are transported tidily in line during processing; a plurality of nozzles14 supplying high pressure jets 15 of poorly mineralized water (aspreviously defined, for example well water) towards the non electricallyconductive elements 2 found on the rack 12; at least one pump 16supplying (at a pressure greater than 60 bar and preferably equal to 70bar) the low mineral content water (contained in a tank not illustratedfor simplicity or obtained directly from a well or other naturalsources) to nozzles 14; and an extraction hood 17 facing the supplynozzles 14 and provided with a suction fan 18 that collect theenvironment air together with most of the water supplied by the nozzles14, so as to prevent water dripping onto the elements 2 after the jets15 have passed through the rack 12 (through the spacing between therollers 13) and “wetting” the elements 2 found on the rollers 13.

In particular, the nozzles 14 supply jets 15 that expand in a conepattern dispersing the water in the air thanks to the drop in pressureat the nozzle, creating in the process a mist (a very fine uniformdispersion of water micro droplets in air) all around the nonelectrically conductive elements 2 traversing on rollers 13, around therollers 13 themselves, and in general around the entire volume betweenthe nozzles 14 and hood 17, which wets and deposits a conductive filmonto the insulating surface of the elements 2, and where excess water issubsequently aspirated by the hood 17 by means of the suction fan 18,which forces the water into a discharge pipe 19. Being a simple mixtureof water and air, the mist aspirated by the hood 17 may be dischargeddirectly into the environment, or treated in order to at least partiallyrecycle the water.

According to the illustrated preferred embodiment, the supply nozzles 14(see FIG. 3) are aligned and spaced along the direction L (FIG. 3),perpendicular to the transport direction D of the non electricallyconductive elements 2 along the plant 1 in general, and on the motorizedrollers 13 rack 12 in particular. Direction L is specificallyperpendicular to direction D.

In the illustrated example, nozzles 14 are placed under the motorizedrollers 13 of rack 12, in order to aim the jets 15 from below upwards;moreover, according to an important aspect of the invention, the nozzles14 are spaced apart vertically from the motorized rollers 13 of rack 12by a fixed amount i.e. a predetermined distance T, so that between therack 12 of motorized rollers 13 and the nozzles 14 there is an emptyspace, indicated by the letter S in FIG. 3.

Preferably, station 3 in plant 1 also comprises a means 20 for varyingin an adjustable manner the predetermined distance T, which isillustrated schematically only in FIG. 3 as holes 20 b and as ahydraulic or pneumatic piston 20 c (any other type of actuator issuitable). According to the non limiting illustration in FIG. 3, therollers 13 are each attached to a gear 21, which is rotated by means ofa gear transmission 23 driven by a motor 24 common to all four rollers13, which therefore rotate synchronously. Rollers 13, transmission 23,and motor 24, together with all the remaining rack 12 parts aresupported by a frame 25 which is part of the support structure of theplant 1, not illustrated for sake of simplicity.

Frame 25 also comprises upright supports 26 eventually provided withholes 20 b spaced apart to one another in the vertical direction; forexample, on the upright supports 26 (only one of which is shown forsimplicity in FIG. 3, however it is clear that at least two arenecessary, one on each side of rack 12) slides a C section beam 27. Thebeam 27 provides support for a pipe 28 laid out along direction L, whichfurther bears upwards the nozzles 14. The nozzles 14 are arranged in aline within a pair of bulkheads 29 also connected to beam 27, the topborder of which defines the distance T of the nozzles 14 from therollers 13. Beam 27 may be activated automatically by actuation means 20c, or displaced manually while system 1 is not operating, and fixed in anew position by inserting specially devised fixing pins or cleats (knownin the art and not illustrated for simplicity) into holes 20 b.

While possessing the above described characteristics, extraction hood 17is not only constructed as illustrated schematically in FIG. 1, but isinstead implemented as an extraction hood 17 b depicted in FIG. 2.

Extraction hood 17 b is defined by a tubular element 30 facing nozzles14 below, provided with a straight longitudinal aspiration slit 31running along the direction L of nozzles 14. Said straight slit 31 isdefined along its length by two V bent edges/rims 32 bending back intotubular element 30 so as to define inside and at the bottom of saidtubular element a water collection suction trap, from which suction trap33 the collected water (which is thus prevented from falling backtowards elements 2) is then removed by the suction fan 18 by means ofsuction. For this purpose, suction fan 18 is arranged to face directlyopposite slit 31.

In this manner, the mist created by nozzles 14, thanks to the emptyspace S between them and rollers 13, in which jets 15 can be dispersedprecisely forming the cited mist, is extracted by hood 17 b through slit31, causing it to travel around elements 2 and rack 12. Subsequently,the water droplets suspended in the air in order to form said mist, areextracted by hood 17 b together with the air where they are suspended; alarge portion of the water finds its way directly into drain 19; thewater droplets that are not directed to drain 19 lose velocity withinpipe 30, which operates as a form of cyclone, and are collected intosuction trap 33, without falling onto elements 2, also thanks to thepresence of the bent edges 32.

Therefore, elements 2 being transported upon rack 12 receive acontrolled amount of water, which they are able to covalently retain,consequently slightly increasing their weight. System 1, and inparticular the elements comprising station 3, is/are sized to cause anincrease in weight of elements 2 traversing station 3 of between 0.15%and 0.30%.

With respect to FIG. 4, where similar or identical details to thosedescribed previously are indicated with the same reference numbers forsimplicity, a plant 1 b is depicted corresponding to a possiblevariation of plant 1 according to the invention, previously described.

Plant 1 b differs from plant 1 in that pre-treatment station 3 isreplaced by a pre-treatment station 3 b again for the purpose ofuniformly wetting in a controlled manner at least the surface 8 to becoated, and preferably each entire non electrically conductive element 2by adsorption and/or deposition of poorly mineralized water which iscovalently retained on elements 2, but in a different manner.

Pre-treatment station 3 b comprises: a superheated steam generator 35that receives a flow of low mineral content water F, as definedpreviously, and heats it in order to create a flow of steam V atapproximately 200° C.; a plurality of nozzles 38 supplying such steam tonon electrically conductive elements 2; and air/steam mixing means 37suitable for creating a flow of air/steam with a specific volume ofbetween 15 and 30 m³/kg directed towards non electrically conductiveelements 2, when these are located on transport device 7.

According to the non limiting example shown, plant 1 b comprises threeseparate transport devices 7, a first device to load elements 2 intostation 3 b, a second device 7 included as an integral component ofstation 3 b, on which elements 2 are found when struck by the air/steamflow, and a third device 7 traversing stations 4 and 6 to deposit thecoated elements 2 onto table 10. Nozzles 38 (as is the case for all ofstation 3 b) are obviously located upstream of electrostatic applicationstation 4 of coating powders 5, and are arranged in an analogous schemeto that of nozzles 14, but positioned so as to operate above elements 2and above transport mechanism 7 on which elements 2 are placed tidilye.g. in series while traversing station 3 b.

Station 3 b further comprises a cooling means 40 for elements 2 blocated immediately upstream of powder coating station 4. Elements 2 bare simply the non conductive elements 2 that retain the poorlymineralized water supplied by said controlled air/steam flow and whichtherefore form a covalent bond (also for system 1 described previously)directly above elements 2, which thus become electrically conductiveelements 2 b.

In the depicted non limiting embodiment, air/steam mixing means 37 andcooling means 40 are defined by two adjacent sections 390 and 391 of atunnel hood 39 arranged above transport mechanism 7 comprised in station3 b at a predetermined distance T2 from transport mechanism 7 and from asuction fan/aspirator 392 assembled on tunnel hood 39 which aspiratesair from the environment into tunnel hood 39 through a slit or gap 393defined by the distance or empty space T2 between the top surface oftransport mechanism 7 on which elements 2/2 b are placed and the bottomborder of tunnel hood 39; said air, together with a large portion of thesteam supplied by nozzles 38, which are arranged inside tunnel hood 39within section 390 distal from coating station 4, is eliminated by meansof a drain 394. Section 391 of tunnel hood 39 is instead next to station4 and is found immediately downstream of nozzles 38 and immediatelyupstream of electrostatic powder coating 5 application station 4.

Within section 390 the steam V supplied by nozzles 38 is mixed with partof the environmental air aspirated through gap 393 within hood 39, wherethe flow of air/steam formed in this manner is directed towards fan 392and hits elements 2, depositing an amount of poorly mineralized wateronto the same, which adheres to surface 8 by means of covalent bonds,“transforming” non conductive elements 2 into conductive elements 2 b.In section 391, the remaining environmental air aspirated into tunnelhood 39 by fan 392 hits electrically conductive elements 2 b, thuseliminating potential excess water and cooling them (elements 2/2 b arein fact heated in section 390 as a result of the heat content of steam Vto a temperature not suitable for carrying out the electrostatic powdercoating 5 deposition).

On the basis of the above description, it is clear that both plants 1and 1 b are suitable for implementing a method for powder coating ofelectrically non conductive elements 2, in particular brake pads,comprising: a pre-treatment phase, where non electrically conductiveelements 2 are made to electrically conduct on at least one of a surface8 thereof to be coated, giving rise to temporarily conductive elements 2b; a deposition phase where coating/painting powder 5 is applied bymeans of an electrostatic field to surface 8 to be coated; and a bakingphase, where coating powder 5 previously applied on elements 2 b ismelted and polymerized in order to create a coating layer 9 on surface 8to be coated.

According to the invention, instead of performing the deposition byspraying using solvents and a conductive primer as in the prior art, thepre-treatment phase consists in uniformly wetting at least surface 8 tobe coated, and preferably each entire non electrically conductiveelement 2, by adsorption and/or deposition of water, preferably poorlymineralized water, as defined previously, in order to produce ameasurable weight increase in the non electrically conductive elements2, which causes said elements 2 to temporarily conduct electrically,giving rise to elements 2 b.

The water retained by the non electrically conductive elements 2 inorder to create elements 2 b, which differ from elements 2 preciselybecause of the presence of water, preferably low mineral content water,adhered by means of covalent bonds to at least surface 8, issubsequently at least partially eliminated (preferably substantiallycompletely removed) during the baking phase.

The poorly mineralized water retained by elements 2 by means of covalentbonds during the pre-treatment phase must possess a chemical compositionsuch that the specific conductivity of such water measured at 20° C. isto be comprised between 1 and 5,000 μS/CM and preferably between 10 and700 μS/cm. Furthermore, the pre-treatment phase is carried out so thateach non electrically conductive element 2 retains such an amount ofwater (adsorbed and/or deposited) as to cause an increase in weight ineach non electrically conductive element 2 of between 0.15% and 0.30%.

By means of plant 1 b, the pre-treatment phase is performed producing,starting from said poorly mineralized water, superheated steam at least200° C., mixing a flow V of said superheated steam with at least aproportion of an air flow A aspirated inside tunnel hood 39 from theexterior in order to generate at nozzles 38 housed in section 390 oftunnel hood 39 a flow of air/steam in a ratio of between 15 and 30m³/kg, and directing said steam/air flow by means of suction fan 392located in section 391 of tunnel hood 39, and therefore placed next tocoating station 4 downstream of nozzles 38, onto the non electricallyconductive elements 2, arranged tidily, e.g. in line on a conveyor belt(mechanism 7). Said pre-treatment phase is immediately followed by acooling phase of non electrically conductive elements 2 and madeelectrically conductive (elements 2 b) because of the poorly mineralizedwater retained with covalent bonds. Said cooling phase is carried outbefore depositing the painting powder 5, in section 391 of the tunnelhood 39, arranged downstream of section 390 where the nozzles 38 arelocated.

On the other hand, by means of plant 1 the pre-treatment phase iscarried out by spraying high pressure poorly mineralized water jets 15onto the non electrically conductive elements 2 in order to create amist around said elements, while extracting the mist through extractionhood 17/17 b.

In this case, jets 15 of said poorly mineralized water are aimed at nonelectrically conductive elements 2 from below, while the elements 2 aremoving on the motorized roller rack 12, above which the extraction hood17 is located.

The effects of the pre-treatment method according to the invention weretested experimentally on a set of samples. A plurality of brake pads ofa type known in the art were produced, utilizing however asbestos-freeorganic friction compound.

Some of the brake pads were treated by means of the above describedstation 3 b with the steam/air mixture and subsequent cooling, measuringtheir weight before the treatment, after coming into contact with theair/steam mixture (wet pads) in section 390 of tunnel hood 39, aftercooling in section 391 of tunnel hood 39, and after baking in tunneloven 6, at the same temperature utilized for traditionally painted pads.The results obtained with the samples are displayed in FIG. 5. As it canbe immediately seen, the charts depicting the change in weight of thevarious samples are completely consistent as a trend, and display anincrease in weight for wet pads, which diminishes after the coolingphase, and which substantially disappears at the end of the bakingphase, proving that the water retained by the brake pads after supplyingthe steam/air mixture is eliminated in the powder coated (painted) pads.Similarly, the continuous line chart displays the weight variation trendfor a coated pad: as it can be seen, the weight of the pad after coolingremains constant, since the weight of the water lost is equivalent tothe weight of the coating (coating powder 5) deposited and baked instations 4 and 6.

Further brake pads were treated in station 3 described previously,displaying the same weight variation trend.

Finally the mean of the weight increase/decrease measurements andelectrical resistance for the “blank” (not coated) brake pads beforewater treatment, after treatment, at the exit of tunnel hood 39, and atthe exit of oven 6 are shown. The results obtained are shown in thecharts in FIG. 6. As it can be seen, in the “wet” state the nonconductive brake pads become conductive, showing mean electricalresistance values between 0.01 and 0.02 Mega Ohm, against a meanelectrical resistance values of non treated pads (not wet) and afterbaking in oven 6 of approximately 1,000,000 Mega Ohm.

Since stations 4 and 6 are identical to those of traditional powdercoating plants devised for treating brake pads obtained with conductivecompounds (containing metal), it is clear that plants 1 and 1 b can bederived from already existing plants, by simply adding station 3/3 b inseries. Furthermore, by simply activating/deactivating stations 3 and 3b, plants 1 and 1 b are suitable for treating brake pads obtained fromboth conductive compounds, and non conductive compounds.

Therefore the aims of the invention are fully met.

The invention claimed is:
 1. A system for application of powder coatingsto a plurality of electrically non-conductive elements, said systemcomprising: a pre-treating station adapted to coat the plurality ofelectrically non-conductive elements to provide a plurality oftemporarily electrically conductive elements; a coating stationdownstream of the pre-treating station having at least one dispensingnozzle that enables painting powders to be electrostatically applied tothe plurality of temporarily electrically conductive elements; apolymerizing station downstream of the coating station that is adaptedto melt and polymerize the painting powders applied to the plurality oftemporarily electrically conductive elements; and at least one conveyordevice extending along a path through the pre-treating station, thecoating station and the polymerizing station, enabling transit of theplurality of the electrically non-conductive elements, wherein thepre-treating station is adapted to provide water to be covalentlyretained on at least one surface of the plurality of electricallynon-conductive elements in order to produce an increase of weight in theplurality of the electrically non-conductive elements and causing theplurality of electrically non-conductive elements to be temporarilyelectrically conductive; the polymerizing station adapted to melt andpolymerize and being further adapted to eliminate at least part of thewater previously retained on the plurality of electricallynon-conductive elements by evaporation; and wherein the pre-treatingstation comprises: a rack having rollers spaced apart from one anotherto enable the plurality of electrically non-conductive elements totransit along the path through the pre-treating station; said rollersare motorized rollers; a plurality of nozzles disposed beneath the rackand spaced apart vertically from the rollers by a fixed distance so thatan empty space is created between the rack and the plurality of nozzles,the plurality of nozzles being effective to supply pressurized jets ofwater that expand in a cone pattern dispersing the water in air due to adrop in pressure in the plurality of nozzles, towards the plurality ofelectrically non-conductive elements which creates a mist which is auniform dispersion of water micro droplets in the air all around theplurality of electrically non-conductive elements, the plurality ofnozzles being arranged aligned side by side along an aligning directionthat is transverse to a forward direction along which the plurality ofelectrically non-conductive elements move on the rack with thepressurized jets of the water from the plurality of nozzles extendingupwardly through the rack, traversing on the rollers, around therollers, and through each of the spacings between each of the rollers towet the plurality of electrically non-conductive elements; at least onepump operatively connected to the plurality of nozzles; and anextraction hood arranged to face the plurality of nozzles and theplurality of electrically non-conductive elements, said extraction hoodbeing configured to aspirate the mist around the plurality ofelectrically non-conductive elements by the plurality of nozzles.
 2. Thesystem according to claim 1, in which: i) the plurality of nozzles andthe at least one pump are adapted to generate the pressurized water jetsat a pressure of at least 60 bar; ii) and the extraction hood comprisesa tubular element provided with a longitudinal aspiration slot facingopenings in the plurality of nozzles, the tubular element provided witha longitudinal aspiration slot running lengthwise in a direction that isarranged parallel to the aligning direction of the plurality of nozzles.3. The system according to claim 2, in which the fixed distance betweenthe plurality of nozzles and the rack can be adjusted, and in which thelongitudinal aspiration slot is defined by a pair of V-folded edgesextending towards the inside of the tubular element so as to delimitwithin and define a water collecting suction trap.
 4. The systemaccording to claim 1, wherein the pre-treating station is adapted toproduce a low mineral content water for dispensing as the uniformdispersion of water micro droplets in the air onto the plurality ofelectrically non-conductive elements.
 5. The system according to claim1, wherein the plurality of the electrically non-conductive elements arebraking elements.
 6. The system according to claim 5, in which each ofthe braking elements include an organic friction compound assembled ontoa metal support.
 7. The system according to claim 6, in which the metalsupport of each braking element is made from iron.
 8. The systemaccording to claim 1, in which the water provided in the pre-treatingstation is a low mineral content water having a conductivity measured at20° C. between 1 and 5000 μS/cm, the low mineral content water beingdevoid of ions that are chemically reactive with iron compounds.